public class Solution {
    public int tribonacci(int n) {
        int[] dp = new int[n + 3];
        dp[0] = 0;
        dp[1] = 1;
        dp[2] = 1;
        for(int i = 0; i <  n; i++) {
            dp[i + 3] = dp[i] + dp[i + 1] + dp[i + 2];
        }
        return dp[n];
    }
    public int waysToStep(int n) {
        int MOD = (int)1e9 + 7;
        // 处理⼀下边界情况
        if(n == 1 || n == 2) {
            return n;
        }
        if(n == 3) {
            return 4;
        }
        int[] dp = new int[n + 1];
        dp[1] = 1;
        dp[2] = 2;
        dp[3] = 4;
        for(int i = 4; i <= n; i++){
            dp[i] = ((dp[i - 1] + dp[i - 2]) % MOD + dp[i - 3]) % MOD;
        }
        return dp[n];
    }
}
